(1) Field of the Invention
The present invention relates to an oxygen sensor. More particularly, the invention relates to an oxygen sensor comprising an oxygen ion conductive solid electrolyte.
(2) Description of the Prior Art
Various oxygen sensors for determining oxygen concentrations in various gases are known in the art. For determination of the oxygen concentration in high temperature gases, an oxygen sensor comprising an oxygen ion conductive solid electrolyte is used most broadly.
As shown in FIG. 1, this oxygen sensor comprises an oxygen ion conductive solid electrolyte 1 molded to have an optional shape and voltage pick-up electrodes 2 and 3 coated on the two main surfaces of the molded solid electrolyte 1, respectively. When one electrode is exposed to an appropriate standard gas (for example, air), if the other electrode is caused to fall in contact with a gas containing free oxygen, oxygen in the gas is converted to an oxygen ion by electrochemical oxidation-reduction reaction. Accordingly, a voltage is generated between both the electrodes, and this voltage corresponds to the concentration of oxygen in the gas.
When two gases 4 and 5 differing in the oxygen concentration are caused to fall in contact with both the surfaces of the oxygen ion conductive solid electrolyte 1, a concentration cell is formed, and a voltage corresponding to the difference of the oxygen concentration is generated between both the electrodes 2 and 3. Accordingly, when a standard gas having a known concentration (in general, air is used) is caused to fall in contact with one surface of the oxygen conductive solid electrolyte and a gas to be tested is caused to fall in contact with the other surface, the oxygen concentration in the gas to be tested can be determined from the voltage generated between the two electrodes.
As the oxygen ion conductive solid electrolyte for an oxygen sensor, solid solutions of ZrO.sub.2, HfO.sub.2, CeO.sub.2 or ThO.sub.2 with at least one member selected from MgO, CaO, and rare earth element oxides such as Y.sub.2 O.sub.3 and Nd.sub.2 O.sub.3 can be used. So-called "stabilized zirconia" such as ZrO.sub.2 --Y.sub.2 O.sub.3 or ZrO.sub.2 --CaO is most popularly used as the oxygen ion conductive solid electrolyte. However, oxygen sensors comprising these oxygen ion conductive solid electrolytes are defective in that when they are used at lower temperatures below 500.degree. C., the overvoltage of the electrode reaction becomes extreme and no good characteristics can be obtained, though they show excellent characteristics at higher temperatures, for example, about 1000.degree. C.
Accordingly, when such conventional oxygen sensor is used for determining the oxygen concentration in, for example, an automobile exhaust gas, good results can be obtained if an automobile is driven at a high speed and the temperature of the exhaust gas is high, but if the driving speed is low and the temperature of the exhaust gas is lowered, no good results can be obtained. Because of such inferior low temperature characteristics, the application range of conventional oxygen sensors of this type is drastically restricted.
Various attempts have heretofore been made to improve such inferior low temperature characteristics. For example, contrivances have been made on the mathod of forming electrodes of a noble metal, and there has been proposed a method in which a special oxide is used as an electrode-constituting material. However, no satisfactory results can be obtained according to these proposals.